For surveying a target point, numerous surveying systems have been known since the antiquity. Spatial standard data recorded here are direction or angle and usually also a distance of a measuring system to the target point to be surveyed, and in particular the absolute position of the measuring system is captured in addition to any existing reference points.
Generally known examples for geodetic surveying systems are theodolites, tachymeters and total stations, which are also referred to as electronic tachymeters or computer tachymeters. A geodetic measurement apparatus of the prior art is described, for example, in the publication document EP 1 686 350. Such systems have electrosensory angle and possibly distance measurement functions, which permit the determination of a direction and distance to a selected target. The angle or distance variables are here ascertained within the internal reference system of the system and must still be linked, if appropriate, to an external reference system for absolute position determination.
With respect to the configuration of the surveying systems, numerous different embodiments are known. For example, modern total stations have microprocessors for digital further processing and storing of captured measurement data. The systems generally have a compact and integrated construction, wherein typically coaxial distance measurement elements and computing, control and memory units are present in a system. In dependence on the level of expansion of the total station, it is additionally possible for a motorization of the targeting and sighting device and—in the case where retroreflectors (for example a 360° prism) are used as target objects—means for automatic target finding and tracking to be integrated. As a human/machine interface, the total station can have an electronic display control unit generally a microprocessor computing unit with electronic data storage means—with a display and input means, for example a keyboard. The display control unit is provided with the electrosensory captured measurement data, such that the position of the target point is ascertainable, optically displayable and storable by the display control unit. Total stations known from the prior art can furthermore have a radio data interface for establishing a radio link to external peripheral components, such as for example to a portable data capturing system, which can be configured in particular as a data logger or field computer.
For sighting or targeting the target point to be surveyed, generic geodetic surveying systems have a telescopic sight, such as for example an optical telescope, as the sighting device. The telescopic sight is generally rotatable about a vertical standing axis and about a horizontal tilting axis relative to a base of the measuring system, such that the telescope can be aligned, by pivoting and tilting, with the point to be surveyed. Modern systems can have, in addition to the optical viewing channel, a camera, which is integrated in the telescopic sight and is aligned, for example, coaxially or parallel, for capturing an image, wherein the captured image can be displayed in particular as a live image on the display of the display control unit and/or on a display of the peripheral system—such as for example the datalogger—used for remote control. The optical unit of the sighting device can here have a manual focus—for example an adjusting screw for adjusting the position of a focusing optical unit—or an autofocus, wherein the focus position is changed for example by way of servomotors. Such a sighting device of a geodetic surveying system is described for example in EP 2 219 011. Automatic focusing devices for telescopic sights of geodetic systems are known for example from DE 197 107 22, DE 199 267 06 or DE 199 495 80. The construction of generic telescopic sights of geodetic systems is illustrated in the publication documents EP 1 081 459 or EP 1 662 278.
Commonly used geodetic surveying systems meanwhile by default have an automatic target tracking function for prisms used as target reflectors (ATR: “Automatic Target Recognition”). To this end, for example a further separate ATR light source and a specific ATR detector, which is sensitive to this wavelength (for example a CCD area sensor), are additionally integrated in the telescope.
Also known are measuring systems which are configured specifically for the continuous tracking of a target point and a coordinative position determination of said point. These can, in particular in the technical field of industrial surveying, generally be combined under the term “laser tracker.” A target point can be represented here by a retroreflective unit (for example a cube prism), which is targeted using an optical measurement beam of the measurement apparatus, in particular a laser beam. The laser beam is reflected in parallel fashion back to the measuring system, wherein the reflected beam is captured using a capturing unit of the apparatus. Here, an emission or reception direction of the beam is ascertained, for example using sensors for angle measurement, which are associated with a deflection mirror or a targeting unit of the system. Additionally, a distance between the measuring system and the target point is ascertained by capturing the beam, for example using time-of-flight or phase difference measurement.
With respect to the configuration of laser trackers as surveying systems, modern tracker systems have increasingly as standard—a sensor for ascertaining an offset of the received measurement beam from what is known as a servocontrol point. This measurable offset can be used to determine a position difference between the center of a retroreflector and the point of incidence of the laser beam on the reflector, and to correct or adjust the alignment of the laser beam in dependence on said deviation such that the offset on the sensor is reduced, in particular is “zero”, and the beam is thus aligned in the direction of the reflector center. By adjusting the laser beam alignment, a continuous target tracking of the target point can take place, and the distance and position of the target point relative to the surveying system can be continuously determined. The adjustment can be implemented in this case using a controlled alignment change of the deflection mirror, which is movable by way of a motor and is provided for deflecting the laser beam, and/or by pivoting the targeting unit which includes the beam-guiding laser optical unit.
For range finding, laser trackers of the prior art have at least one distance meter, wherein the latter can be configured for example as an interferometer. Since such range finding units can measure only relative distance changes, what are known as absolute distance meters, in addition to interferometers, are incorporated in modern laser trackers. By way of example, such a combination of measurement means for range determination is known from the product AT901 from Leica Geosystems AG. The interferometers used in this context for the distance measurement primarily use—on account of the large coherence length and the measurement range thus made possible—gas lasers as light sources, in particular HeNe gas lasers. The coherence length of HeNe lasers can in this case be a few hundred meters, such that with relatively simple interferometer construction it is possible to achieve the ranges required in industrial metrology. A combination of an absolute distance meter and an interferometer for range determination using a HeNe laser is known for example from WO 2007/079600 A1.
In many geodetic applications, points are surveyed by placing specifically embodied target objects (for example surveying poles) at the target point. Said target objects usually comprise a plumb pole with a reflector (for example a 360° prism) for defining the measurement section or the measurement point. In the case of such surveying tasks, data, instructions, language and further information are typically transmitted between the target object—in particular a portable data capturing system at the target object—and a central measuring system for controlling the measurement operation and for fixing or registering measurement parameters. Examples of such data are identification information for the target object (for example the type of the prism used), the inclination of the plumb pole, the height of the reflector above the ground, reflector constants or measurement values such as temperature or air pressure. Said information or situation-related parameters is/are necessary in order to allow highly precise targeting and surveying of the measurement point that is defined by the plumb pole with a prism.
Even in industrial surveying, specifically embodied target objects or auxiliary measuring instruments for surveying a measurement point, in particular a plurality of measurement points, are used. These target objects include contactlessly measuring sensors (for example mobile optical scanning units) and also what are known as probing tools which are positioned by way of their contact point on the measurement point on an object and thus permit surveying of said target point.
By interaction of an above-mentioned measurement system with a reflector unit that is used in each case, it is possible to reliably and precisely determine the position of said reflector (on the auxiliary measuring instrument). For measurements of one or more specific target points, however, the position of the reflector alone is usually not sufficient, since the reflector does not directly indicate the target point to be determined, but said target point is surveyed using the target object or the auxiliary instrument (for example a plumb pole in geodesy).
A determination of the spatial orientation or an inclination with respect to in each case relevant spatial directions of the respective auxiliary instrument is thus additionally necessary in order to derive the position of the target point, which is to be determined using the instrument, together with the determined position of the reflector arranged on the auxiliary instrument. Such an orientation can be determined for example by means of an inclination sensor or an IMU (inertial measurement unit), which is provided in a defined position and location relative to the reflector, or—as is typically used for laser trackers—by means of markings arranged on the auxiliary instrument, with the positions of the markings on the contact probe instrument being precisely known and the orientation being determined by image processing of an image in which said markings are captured with positional reference. The image can be captured by an image capture unit on the part of an above-mentioned surveying system.
A disadvantage of these solutions according to the prior art is not only the limited precision owing to inclination sensors, which are typically embodied as liquid sensors, or owing to the IMU, in particular because of the drift of the sensors provided, in particular over a relatively long period of time, for the inclination or orientation determination. Additionally, the markings provided for orientation determination include an additional error source, that is to say that, by way of example, if one or a plurality of markings are partially covered, a corresponding orientation determination may still be possible, but the orientation can be determined only with limited accuracy.
Even a solution according to EP 1 200 853, in which a prism has a through-passage surface for the measurement beam and the latter is incident in part directly on a sensor, has similar disadvantages with respect to achievable accuracies on account of undefined beam guidance and shaping. Precise determination of the position of the radiation on the sensor is achieved only to a severely limited degree, since this determination accuracy depends considerably on the measurement distance and the beam quality. In addition, the structural configuration is configured to be very complex and spatially demanding (prism in front of the sensor or integration of the sensor in the reflector). On account of structure-related, reflector-internal reflections, it is possible even during a distance measurement to this unit, for significant measurement errors during the distance determination to this unit to occur.